1. Field of the Invention
The present invention relates to a laser drive circuit for driving a laser device for emitting laser light and to a method for driving a laser. More particularly, this invention relates to recording/reading equipment for recording video information onto optical disks such as CDs, DVDs and Blu-ray Disks (BDs) or reading information recorded on an optical disk.
2. Description of Related Art
Semiconductor laser devices usually are used to record recording marks on optical disks. In order to prevent the recording marks from deforming into oval shapes, laser devices are configured to emit pulsed light so as to control the heat during recording. With the increase of recording speed in recent years, the pulse widths have become shorter. Along with this trend, demand is increasing for laser drive circuits that have short rising and falling times and are capable of faster switching.
An example of a conventional laser drive circuit is disclosed in JP 2003-324339A. FIG. 11 is a circuit diagram showing a configuration of a laser drive circuit disclosed in JP 2003-324339A. As shown in FIG. 11, p-channel type (hereinafter simply referred to as “p-ch”) MOS transistors 1116 and 1117 and n-channel type (hereinafter simply referred to as “n-ch”) MOS transistors 1112 and 1113 are connected in series between a power source Vcc and ground. A laser diode 1109 capable of emitting laser light is connected between an IC output terminal 1108 and the power source Vcc.
FIG. 12 shows the signal waveforms of the components of the laser drive circuit shown in FIG. 11. FIG. 12 shows only a falling edge of an input pulse 1120. FIG. 12(a) shows the waveform of the input pulse 1120. FIG. 12(b) shows the waveform of the output current of the transistor 1112. FIG. 12(c) shows the waveform of a correction pulse 1121. FIG. 12(d) shows the waveform of the output current of the transistor 1116. FIG. 12(e) is the waveform of a current applied to the laser diode 1109.
As shown in FIG. 11, a voltage from a bias voltage source 1118 is applied to the gate of the n-chMOS transistor 1112. Between the source of this n-chMOS transistor 1112 and ground, the n-chMOS transistor 1113 is connected in series. The n-chMOS transistor 1112 switches on/off the output current at high speed (see FIG. 12(b)), in accordance with the input pulse 1120 applied to the gate of the n-chMOS transistor 1113 (see FIG. 12(a)).
A voltage from another bias voltage source 1119 is applied to the gate of the p-chMOS transistor 1116. The p-chMOS transistor 1116 is connected to the output terminal 1108. Between the source of the p-chMOS transistor 1116 and the power source, the p-chMOS transistor 1117 is connected in series. The correction pulse 1121 (see FIG. 12(c)) is applied to the gate of the p-chMOS transistor 1117 in accordance with the input pulse 1120. When the n-chMOS transistor 1113 is turned off, the p-chMOS transistor 1116 discharges current as shown in FIG. 12(d), whereby the falling time of a current applied to the laser diode 1109 can be shortened as shown in FIG. 12(e).
FIG. 12(b) shows a current waveform without the correction by the correction pulse. The falling time is indicated by t11. FIG. 12(e) shows a current waveform with correction by the correction pulse. The falling time is indicated by t12. The falling times t11 and t12 satisfy t11>t12.
However, according to the configuration disclosed by JP 2003-324339A, the voltage at the laser connecting terminal 1108 may become higher than the power source voltage of the drive circuit due to the characteristics and variations of the laser diode 1109. If the terminal voltage of the laser connecting terminal 1108 is higher than the drain voltage of the p-chMOS transistor 1116, the p-chMOS transistor 1116 cannot discharge the current (see FIG. 12(d)), and the output current from the transistor 1112 shown in FIG. 12(b) cannot be corrected. As a result, the falling time of the pulse cannot be shortened. In other words, the current shown in FIG. 12(b) is outputted directly from the transistor 1112.